Phloem Proteomics Reveals New Lipid-Binding Proteins with a Putative Role in Lipid-Mediated Signaling.

Global climate changes inversely affect our ability to grow the food required for an increasing world population. To combat future crop loss due to abiotic stress, we need to understand the signals responsible for changes in plant development and the resulting adaptations, especially the signaling molecules traveling long-distance through the plant phloem. Using a proteomics approach, we had identified several putative lipid-binding proteins in the phloem exudates. Simultaneously, we identified several complex lipids as well as jasmonates. These findings prompted us to propose that phloem (phospho-) lipids could act as long-distance developmental signals in response to abiotic stress, and that they are released, sensed, and moved by phloem lipid-binding proteins (Benning et al., 2012). Indeed, the proteins we identified include lipases that could release a signaling lipid into the phloem, putative receptor components, and proteins that could mediate lipid-movement. To test this possible protein-based lipid-signaling pathway, three of the proteins, which could potentially act in a relay, are characterized here: (I) a putative GDSL-motif lipase (II) a PIG-P-like protein, with a possible receptor-like function; (III) and PLAFP (phloem lipid-associated family protein), a predicted lipid-binding protein of unknown function. Here we show that all three proteins bind lipids, in particular phosphatidic acid (PtdOH), which is known to participate in intracellular stress signaling. Genes encoding these proteins are expressed in the vasculature, a prerequisite for phloem transport. Cellular localization studies show that the proteins are not retained in the endoplasmic reticulum but surround the cell in a spotted pattern that has been previously observed with receptors and plasmodesmatal proteins. Abiotic signals that induce the production of PtdOH also regulate the expression of GDSL-lipase and PLAFP, albeit in opposite patterns. Our findings suggest that while all three proteins are indeed lipid-binding and act in the vasculature possibly in a function related to long-distance signaling, the three proteins do not act in the same but rather in distinct pathways. It also points toward PLAFP as a prime candidate to investigate long-distance lipid signaling in the plant drought response.

Long-Distance Lipid Signaling and its Role in Plant Development and Stress Response.

Lipids are important signaling compounds in plants. They can range from small lipophilic molecules like the dicarboxylic acid Azelaic acid to complex phosphoglycerolipids and regulate plant development as well as the response to biotic and abiotic stress. While their intracellular function is well described, several lipophilic signals are known to be found in the plant phloem and can, thus, also play a role in long-distance signaling. Mostly, they play a role in the pathogen response and systemic acquired resistance. This is particularly true for oxylipins, dehydroabietinal, and azelaic acid. However, several phospholipids have now been described in phloem exudates. Their intracellular function as well as implications and a model for long-distance signaling are discussed in this chapter.

Gene capture by Helitron transposons reshuffles the transcriptome of maize.

Helitrons are a family of mobile elements that were discovered in 2001 and are now known to exist in the entire eukaryotic kingdom. Helitrons, particularly those of maize, exhibit an intriguing property of capturing gene fragments and placing them into the mobile element. Helitron-captured genes are sometimes transcribed, giving birth to chimeric transcripts that intertwine coding regions of different captured genes. Here, we perused the B73 maize genome for high-quality, putative Helitrons that exhibit plus/minus polymorphisms and contain pieces of more than one captured gene. Selected Helitrons were monitored for expression via in silico EST analysis. Intriguingly, expression validation of selected elements by RT-PCR analysis revealed multiple transcripts not seen in the EST databases. The differing transcripts were generated by alternative selection of splice sites during pre-mRNA processing. Selection of splice sites was not random since different patterns of splicing were observed in the root and shoot tissues. In one case, an exon residing in close proximity but outside of the Helitron was found conjoined with Helitron-derived exons in the mature transcript. Hence, Helitrons have the ability to synthesize new genes not only by placing unrelated exons into common transcripts, but also by transcription readthrough and capture of nearby exons. Thus, Helitrons have a phenomenal ability to "display" new coding regions for possible selection in nature. A highly conservative, minimum estimate of the number of new transcripts expressed by Helitrons is ~11,000 or ~25% of the total number of genes in the maize genome.